1. Field of the Invention
The present invention relates generally to a technique for detecting delay times of multiple propagation paths for despreading a spread spectrum signal, and more specifically to a path selection technique useful for detecting delay times of propagation paths which are so closely spaced apart that they cannot be easily separated from each other with clear distinction.
2. Description of the Related Art
Mobile communication system are characterized by multipath propagation of transmitted signals. In a spread spectrum or CDMA (code division multiple access) system, RAKE combining technique is usually employed for combining signals arriving on multiple propagation paths. This technique is used to improve the quality of reception with a minimum level of transmission power, and hence to achieve high utilization efficiency of frequency resource. For RAKE combining it is important to select propagation paths which yield high autocorrelation values.
In the prior art path selection technique, a “path range” is defined, as shown in FIG. 1, for a selected propagation path as a guard interval to protect it from being mistakenly detected as multiple paths. In a stringent multipath environment, strong signals of different propagation paths may arrive within the same chip interval. However, if multiple strong signals arrive within the same guard interval, they cannot be distinguished and separated from each other. Signals that yield high autocorrelation values may be lost and maximal-ratio combining loses its significance. If one of two strong signals is lost, the maximal-ratio combined signal would be degraded by 3 dB.